Auditory brain stem responses evoked by lateralized clicks: is lateralization extracted in the human brain stem?

The dependence of binaurally evoked auditory brain stem responses and the binaural difference potential on simultaneously presented interaural time and level differences is investigated in order to assess the representation of stimulus lateralization in the brain stem. Auditory brain stem responses to binaural click stimuli with all combinations of three interaural time and three interaural level differences were recorded from 12 subjects and 4 channels. The latency of Jewett wave V is shortest for zero interaural time difference and longest for the trading stimuli. The amplitude of wave V is largest for centrally perceived stimuli, i.e., the diotic and trading stimuli, and smallest for the most laterally perceived stimuli. The latency of the most prominent peak of the binaural difference potential DN1 mainly depends on the interaural time difference. The amplitude of the components of the binaural difference potential, DP1-DN1, depends similarly on stimulus conditions as wave V amplitude in the case of the binaural stimuli: smallest amplitudes are found for the most lateral stimuli and largest amplitudes for central stimuli. The results demonstrate that interaural level and time differences are not processed independently. This supports the hypothesis that directional information in humans is already extracted and represented at the level of the brain stem.